Integration of windmills with the existing power system depends on the size and structure of concrete power system and on the capacity of links with neighbouring systems.

Until all the fluctuations of wind power can be compensated with the hydro power plants, the integration of windmills does not trouble the existing system too much and the environmental gain is linearly proportional to the produced amount of electricity.

If the power systems contains only thermal power plants or if the installed capacity of windmills exceeds the regulation capacity of hydro plants:

As the CHP plants usually follow the thermal load, the condensing power plants must participate in the compensation of wind power fluctuations.

Large condensing units cannot be switched on and off frequently and for a short period and their speed of increasing and decreasing of power is limited.

Most suitable thermal plants for the load regulation and fast reserve capacity are the gas turbines.

If someone wants to introduce large amount of wind power then the power regulating range and speed of the existing plants must be also extensive.

Operating a thermal plant with and without the need to compensate the fluctuations of wind power is similar to the running of a car in the city and on the highway, respectively. Fuel consumption of a car can be even double in the city comparing with the highway.

The thermal power production without wind generators is equal to the load and it is distributed among the thermal power plants according to the optimality criterion and using static input-output characteristics.

When the wind power appears in the system, thermal power stations have to keep constantly additional spinning reserve capacity equal to the maximum total power of windmills. This makes the thermal plants run inefficiently and increases fuel consumption (emissions).

Under the fast changes of wind power, the real fuel consumption will increase even higher. The actual operation points of thermal plant will form a curve that is similar to a hysteresis loop. This is the dynamic fuel consumption curve.

Denmark exports wind generated electricity [slide 8 shows that a greater proportion is exported as more wind-generated power is produced, approaching 90% at 2000 MWh/h]

Conclusions [slide 21]

Participation of thermal power plants in keeping the reserve capacity for wind turbines and in compensation of the fluctuations of wind power increases the fuel consumption and emissions substantially.

Linear methods of calculation of emission reductions from wind energy use cannot consider this increase and therefore special methods for correct accounting of environmental gain have to be elaborated.